In railroad and mass and/or rapid transit operation, the braking action is controlled by the operator or trainman in the cab of the lead vehicle and locomotive of a train. In the past, the cab included a brake valve having either a 30 type of desk-style console or a traditional 26 style control stand with each style employing two brake handles for controlling of the braking of the train.
The automatic or train control is accomplished by rotation of a first brake handle through approximately the same angle of rotation on either the 26 or 30 style brake valve. The rotational movement of the automatic brake handle from a release or running position to a minimum service brake position creates a step function resulting in a pressure change in the brake pipe consistent with a minimum brake pipe reduction. Further rotation of the automatic brake handle through the braking range provides a complete variable control of the brake pipe up to the full service brake position. If rotation of the handle is continued through the full service position, it will move to a suppression position, to an over-reduction zone, and finally to an emergency position. A positive mechanical detent is provided for each position to assist the engineman by providing him with a sense of feel for the distinct braking positions.
The control of the locomotive or independent brake is accomplished by rotational movement of a second brake handle. The rotation of the independent brake handle provides a complete variable control from a release position to a full independent application position.
In practice, the cab control unit is arranged and designed to be a man-to-machine interface and is strictly an electrical/electronic device which has no pneumatic connections. An associated microcomputer contains the intelligence to issue and interpret brake commands initiated from the cab control unit to the pneumatic control unit. As noted above, the brake commands emanating from the cab control unit are dependent upon the given positions of the automatic and/or independent brake handles. In practice, the angular positions of the brake handles are monitored and sensed by rotary absolute encoders which are mechanically coupled to associated rotary shafts. In order to accurately detect the discrete angular brake positions, it is necessary to establish a zero or home position for both of the brake handles. In the past, the zero position was mechanically established during the assembly of the equipment. This type of mechanical initialization or set-up of an encoder is both expensive and time-consuming. In addition, a railroad environment is susceptible to a wide variation of temperatures and is exposed to extreme vibrations and agitations which can adversely affect the initial zero position setting.